Monitoring fabric properties

ABSTRACT

The present invention relates to a novel method and apparatus for determining the periodicity of a changeable characteristic of a textile fabric. The method includes the steps of sensing a property related to the characteristic at pairs of positions which are spaced apart by a distance, S, along a length of the fabric and generating signals representative of the magnitude of the property at the positions. The generated signal values are stored and the products of the signals generated at each pair of positions are summed in accordance with the formula: 
     
         ΣX.sub.(y) ·X.sub.(y+s) 
    
     from y=O to y=Y 
     where the parameters are defined in the specification. These steps are repeated for different dimensions, S. The value of S at which the summation of the signals is a maximum is determined and used to generate an output signal representing the value of the periodicity. The method provides information useful for controlling fabric treatment processes such as stentering or compacting, and in particular, for counting the courses of a fabric.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the determination of the periodicity of acharacteristic of a textile fabric which changes in a repeating fashionalong the length of the fabric. Information relating to such acharacteristic can be used to control fabric treatment processes, suchas stentering or compacting, for example, to adjust the number ofcourses of the fabric per unit length to a desired value.

The term "characteristic" is taken to refer to any feature of the fabric(irrespective of whether the fabric is knitted or woven) which repeatsitself. For example the feature may be a structural feature such as thecourses or warp threads of the fabric, or a repeating pattern such asstitches, holes (in lace for example), dyed regions or differentcoloured threads, or some other structural feature of the fabric. Forsimplicity, the invention will be described with reference to countingthe courses of a fabric but the present invention is intended to coverother uses, where the context fits.

The present invention is particularly useful for counting the courses ofa fabric. The course count of a woven fabric is the number of picks in aunit length of the fabric, and the course count of a knitted fabric isthe number of courses in a unit length of the fabric.

2. Description of Prior Art

In some processes for the treatment of fabrics, such as stentering usedto stretch or shrink the fabric or compacting (used to shrink thefabric) to obtain uniformity of spacing of the courses and wales (orwarp and weft threads) it is necessary to adjust the process parametersto compensate for variations in the fabric entering the processequipment.

In a stenter, the fabric is stretched or overfed as it is passed througha heating zone on pin chains. This is usually achieved by controllingthe speed of rotation of rollers over which the fabric passes as itenters and leaves the heating zone and the fabric is held taut on thepin chains to achieve uniform density.

In both of these processes, it is very difficult to count all thecourses accurately and to use this count to control the rate of feed ofthe fabric into, and out of, the stenter or compactor. Not only are thecourses irregularly spaced but the fabric may also be puckered, folded,loose or taut.

A previous method of determining course count has employed aphotoelectric cell to measure the transparency of the fabric along itslength and means for measuring the peak amplitudes of the response curveof the cell thus obtained. Such curves are very irregular in shape andfor many purposes, the reliability of the period measurements obtainedin this way is not sufficiently high.

In the case of a course count determination, a typical signal willcomprise one or more periodic components directly related to the coursecount, together with other random and/or repetitive components which maycompletely mask the course count component, at intervals.

The basic course count frequency may be difficult to measure by thisknown method of counting the signal peaks above a chosen threshold levelsince:

(a) peaks above the threshold level may be generated by a combination ofunrepresentative signals and the basic signal, and

(b) the basic signal may not be present all the time (for example,because the expected apertures in the fabric are obscured).

Furthermore the maximum accuracy of periodicity measurement by thismethod is determined by the number of courses counted in any one samplelength.

It is an object of the present invention to improve the reliability ofdeterminations of the periodicity of a characteristic of a textilefabric such as its course count.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a method ofdetermining the periodicity of a changeable characteristic of a textilefabric is characterised by the steps of:

(a) sensing a property related to the said characteristic at pairs ofpositions which are spaced apart by a distance, S, along a length of thefabric,

(b) generating signals representative of the magnitude of the saidproperty at the said positions,

(c) summing the products of the signals generated at each pair ofpositions in accordance with the formula:

    →x.sub.(y) ·r.sub.(y+S)

from y=0 to y=Y

where x.sub.(y) represents the value of the said property at a positiony along the fabric, and r.sub.(y+S) represents either the value of thesaid property at a position (y+S) along the fabric, or the value ofanother regularly varying function, at the position (y+S) along thefabric,

(d) repeating steps (a) to (c) for different dimensions, S, and

(e) determining the value of S at which the summation of the signals isa maximum and using this value of S to generate an output signalrepresenting the value of the periodicity.

The present invention is based on the use of correlation methods todetect periodicity in a composite signal which may, for example bederived from the structure variations of a fabric in the direction ofmovement of the fabric. The periodicity may be used to control, forexample, a stenter feed mechanism to correct course frequency errors.

One possible procedure in accordance with the invention is based onanalysis of all, or part, of an auto-correlation function of thecomposite signal.

The auto-correlation function is defined as: ##EQU1## In the case of acourse count determination in a fabric: x.sub.(y) is a function xrepresenting the composite signal at a position y along the fabric

x.sub.(y+S) is a function x representing the composite signal at aposition y+S along the fabric

Y=length along the fabric

S=an interval of length along the fabric.

The function T.sub.(y) will exhibit peaks at values of S correspondingto the periods and multiples thereof of repetitive events in theoriginal composite signal x.sub.(y). In the case of a fabric, one ofthose periods will be due to the "course frequency" or a multiple of"course frequency".

Another possible procedure is based on a cross-correlation functionanalysis using the integral: ##EQU2## where r.sub.(y+S) is a periodicreference function, for example a sine function.

Peaks will occur in the cross-correlation function T.sub.(y) when theperiod of the reference signal is equal to a period occurring in thecomposite signal x.sub.(y). Hence by evaluating T.sub.(y) over a rangeof reference signal periods covering a predetermined expected period inthe composite signal x.sub.(y), an actual period in x.sub.(y) can befound.

Since a sufficiently accurate assessment of periodicity in relativelysmall samples can be very reliably made using correlation methods, themethod can be used to control machinery such as a stenter, to reducecourse frequency variations in the processed fabric to a more acceptablelevel. This can be achieved by, for example, measuring the course periodof the cloth being fed to the stenter and controlling the rate of feedto the stenter pin-chain to correct deviations from nominal courseperiod.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be further described, by way of example, withreference to the accompanying drawing, which shows diagrammatically astenter control system according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The apparatus illustrated comprises a stenter 10 having a pin-chain 11carried on rollers 12 and 13, a rotary brush 14 to press fabric 15 ontothe pin-chain 11, and a heating chamber 16. The roller 12 (and thus thepin-chain 11) is driven by a shaft 17 from a gear box 18 which is drivenby a main drive motor 19. A further output shaft 20 of the gear box 18drives a set of coupled feed rollers 23 which feed the fabric 15, theshaft 20 being connected through a variable speed gear box 24 to a shaft25 which is connected to one of the rollers 23 through further gearing26. A drive shaft 27 also connected to the gearing 26 drives a pair offeed rollers 28, in nip relationship, at the same circumferential speedas the roller 23. The rotary brush 14 is drivably coupled to the feedrollers 23 so that it is driven at the same speed as those rollers.

The variable speed gear box 24 is controlled by an electric motor 31 onwhose shaft 32 is mounted a position sensor 33. A light source 34 isarranged to direct a beam of light at the fabric between the feedrollers 28 and the feed rollers 23. A photo-detector 35 is located onthe opposite side of the fabric to receive light from the source 34. Theoutput of the photo-detector 35 is supplied to a computer 36. The outputof the shaft position sensor 33 is also supplied to the computer 36 andthe output of the computer 36 is fed to a motor controller 37 whichcontrols the motor 31.

The computer 36 stores and analyses the composite signal from thephoto-detector 35 and computes the course count period. This is comparedwith the required periods entered via a computer keyboard (not shown)and any difference causes the computer to change the gear ratio of thegear box 24, through the controller 37 and motor 31, so as to correctthe error by adjusting the rate of feed of fabric 15 to the pin-chain 11of the stenter 10.

Referring to the drawing, the photo-detector 35 may be arranged tomeasure either the transparency or reflectivity of the fabric 15; thechoice being dependent on the opacity of the fabric. The area and shapecovered by the photo-detector 35 is chosen to effect the degree ofsignal averaging deemed appropriate for the resolution required. A lenssystem comprising a part-spherical lens 38 and part-cylindrical lens 39focuses the light onto the photo-detector 35. The use of thepart-cylindrical lens defocuses the wales and accentuates the courses.

The continuous signal from the photo-detector 35 is sampled anddigitised at intervals of fabric length in the direction of fabricmovement. The intervals must be sufficiently small to resolve thevariations in the photo-detector signal. The digitised values are storedin order of occurrence in the computer memory (e.g. in an arrayschematically indicated at 42). When the sample is fully digitised, thevalues are read off in pairs, the members of each pair being separatedby a fixed number of values in the array corresponding to a length S offabric. Starting with a pair at array values O and (O+S). the members ofeach pair are multiplied together for values between y and (y+S). Thatis, if the storage array containing the digitised signal values iscalled A, and has K elements, then A.sub.(n) is multiplied byA.sub.(n+S) for all values of n from n=1 to n=(K-S), and the productssummed for each of a range of values of S. This is repeated for a rangeof values of S, which includes the expected value of the periodicity ofthe course count. If the values of the product sums were to be plottedagainst S, any periodicities in the original signal would show as peaksin the curve, the value of S at these points indicating the period.Since the approximate value of the required period in the originalsample is known, a precise value of the actual period of the coursecount can be obtained by detecting the peak value of the product sum inthe region of S corresponding to the required period.

The measurement of periodicity of the course count is thus obtained bycomparing the values of the product sums in the computer and choosingthe maximum value in the range close to the expected value of S whichhas been sampled. This value of the periodicity is then compared in thecomputer with the value of the course count required in the finishedfabric and the difference signal produced is used, as mentioned above,to control the variable speed gear box 24 through the controller 37 andmotor 31. The shaft position sensor 33 feeds back to the computer 36 theactual adjustment of the gear box 24 so that the computer can recognisewhen the correct ratio adjustment has been achieved in the gear box. Thespeed with which the fabric 15 is fed to the stenter 10 is thus adjustedand therefore the degree of stretch applied to the fabric 15 in feedingit to the stenter 10. The course count of the fabric 15 can thus beadjusted to the required value.

Where the photo-detector 35 is located in advance of the rollers 23(indicated with continuous lead lines) a delay in making the adjustmentcan be achieved in the computer 36 to allow for the fact that themeasurement of the actual periodicity in the fabric is determined by thephoto-detector 35 in advance of the position between the feed rollers 23and the pin-chain 11 at which the degree of stretch in the fabric 15 iscontrolled, so that the appropriate adjustment is applied to the correctlength of fabric.

The summation of the products A.sub.(n) ·A.sub.(n+S) effected in thecomputer 36 is equivalent to performing the summation:

    Σx.sub.(y) ·x.sub.(y+S)

from y=0 to y=Y, and the following procedures in the computer describedabove amount to determining for what value of S the summation is amaximum.

If the cross-correlation procedure is to be used, a desired periodicfunction, for example a sine wave, is stored in the computer memory andthe product computed is then A.sub.(n) ·r.sub.(y+S) where r is theperiodic function chosen for the cross-correlation. This product issummed from n=0 and y=0 to y=Y where the values of y are chosen tocorrespond to the values of n, and Y corresponds to n=K-S using thenotation set out above.

The photo-detector 35, that is the point at which the signalrepresentative of the characteristic to be used for control is obtained,may be located either upstream (shown with solid lead lines) ordownstream (shown with dashed lead lines) of the correction mechanism(in this case the feed rollers 23 to the stenter). The advantage ofgenerating the signal downstream of the correction mechanism is that theerror in the characteristic to be controlled, in this case the courseperiod, will be small and the range in the values of S for whichsummations must be carried out will thus be reduced. A consequence ofthis is that the number of computations required to find the relevantpeak will be reduced. Monitoring the relevant characteristics after thecontrol region (where the correction mechanism is located) entails thatno delay should be introduced between the completion of the computationand the effecting of the control operation.

In order to compute the accuracy of a course count in a fabric with anaccuracy of ±0.5% it is necessary to sample an opacity signal at a rateof 200 samples per course and this may require sampling and digitisingrates which are difficult to achieve. Adequate signal information can bederived using far fewer samples from the signal curve and generatingintermediate values between the samples by interpolation between thesample values. This technique requires fewer analogue-to-digitalconversion operations than digitising the same number of sample valuesof the curve.

Thus, when producing values related to the magnitude of a property inorder to carry out the summation mentioned above, some of those valuesmay be derived directly from a signal curve representing the propertysensed and others may be derived, by means of intermediate computations,from values of the property sensed which themselves are derived directlyfrom the signal curve. Interpolation between pairs of directly derivedsignal values is a convenient form of intermediate computation.

I claim:
 1. A method of determining the periodicity of a changeablecharacteristic of a textile fabric characteristic by the steps of:(a)sensing a property related to the said characteristic at pairs ofpositions which are spaced apart by a distance, S, along a length of thefabric. (b) generating signals representative of the magnitude of thesaid property at the said positions, (c) storing the generated signalvalues, summing the products of the signals generated at each pair ofpositions in accordance with the formula:

    Σx.sub.(y) ·X.sub.y+S)

from y=0 to y=Y where x.sub.(y) represents the value of the saidproperty at a position y along the fabric and X.sub.(y+S) representseither the value of the said property at a position (y+S) along thefabric, or the value of another regularly varying function at theposition (y+S) along fabric, (e) repeating steps (a) to (d) fordifferent dimensions, S, and, (f) determining the value of S at whichthe summation of the signals is a maximum, and using this value of S togenerate an output signal representing the value of the periodicity. 2.A method according to claim 1 wherein step (c) comprises the step ofderiving the product of each pair of signals.
 3. A method according toclaim 1, wherein the value of the periodicity that corresponds to thevalue of S at which the sum of the pairs of signals is a maximum, iscompared with a predetermined periodicity, and the difference is used togenerate a difference signal which is used to affect process conditionsto which the fabric is subjected, and thereby tend to reduce the saiddifference.
 4. A method according to claim 1, wherein the characteristicis the count of the number of courses of the fabric.
 5. A methodaccording to claim 4, wherein the property monitored is the opticaltransparency of the fabric, and the signals generated at each of thepositions is representative of the courses which pass an opticaltransparency monitor.
 6. A method according to claim 1, wherein theanalysis of the signals generated in step (b) is based on anauto-correlation function defined by the integral ##EQU3## where:x.sub.(y) is a function x representing a composite signal at a positiony along the fabric,x.sub.(y+S) is a function x representing thecomposite signal at a position y+S, Y=length along the fabric, and S=aninterval of length along the fabric.
 7. A method according to claim 1,wherein the analysis of the signals generated in step (b) is based on across-correlation function defined by the integral: ##EQU4## where:x.sub.(y) is a function, representing a composite signal at a position yalong the fabric,Y=length along the fabric, S=an interval of lengthalong the fabric and, r.sub.(y+S) is a periodic reference function.
 8. Amethod according to claim 6, wherein the output signal is used tocontrol machinery to reduce variations in the course frequency of aprocessed fabric.
 9. Apparatus for carrying out a method of determiningthe periodicity of a changeable characteristic of a textile fabric, saidapparatus comprising, sensing means for sensing a property related tosaid characteristic at pairs of positions which are spaced apart by adistance, S, along a length of the fabric, a generator for generatingsignals representative of the magnitude of the said property at the saidpositions, storing means for storing the generated signal values,summation means for summing the signals generated at each pair ofpositions in accordance with the formula:

    Σx.sub.(y) ·X.sub.(y+S)

from y=0 to y=Y where x.sub.(y) represents the value of the saidproperty at a position y along the fabric and X.sub.(y+S) representseither the value x of the said property at a position (y+S) along thefabric or the value of another regularly varying function at theposition (y+S) along the fabric, and output means for determining thevalue of S at which the summation of the signals is a maximum and usingthis value of S to generate an output signal representing the value ofthe periodicity.
 10. Apparatus according to claim 9, wherein thesummation means includes means for deriving the product of each pair ofsignals.
 11. Apparatus according to claim 9, further including acomparator for comparing a first signal indicative of the value of theperiodicity that corresponds to the value of S at which the sum of thepairs of signals is a maximum, with a second signal indicative of apredetermined periodicity, said comparator being operable to generate anoutput signal indication of the difference between the first and secondsignal which is used to affect process conditions to which the fabric issubjected, and thereby tend to reduce the said difference.
 12. Apparatusaccording to claim 9, wherein optical means (34, 35, 38, 39) is providedfor monitoring the optical transparency of a fabric characterized bycourses, and the signals generated at each of the positions isrepresentative of the courses which pass the optical means (34 35, 38,39).